The present invention relates to continuously variable transmissions.
A transmission is used to match the speed and torque of a rotating load with that of the motor or engine driving it. The driven end is designated as the input while the shaft or member attached to the load is designated as the output.
In vehicular applications, the input speed is always higher than the output speed of a transmission, while in some industrial applications where high speed is required for an operation, the opposite is sometimes true. In any case, a transmission has an input and an output and has control over the ratio of rotational speed between them.
Gears have traditionally been used to achieve these speed ratios. Multiple gear sets are used within a transmission if a variety of fixed ratios is required. A more desirable device would offer the user the ability to continuously vary the speed ratio between input and output over a wide range. Continuously variable transmissions (CVT""s) have been designed to achieve this result.
A wide variety of geometric alternatives and driving members have been tried over the years. Disk/disk, ball drives and belt drives have been used in CVT""s. Some designs used sliding friction while others used rolling friction between members to transmit torque from input to output. While gears use mechanical interlocking to prevent slippage, friction drive elements are subject to slippage, and the size of the elements themselves must be increased or the contact forces between them must be increased (or both) to provide adequate torque capability. The measures to prevent slippage work against the dual goals of achieving compactness and low component wear. This has been the principle impediment to the broad application of CVT""s in heavy-duty applications.
It is therefore an object of the present invention to provide a reliable, positive, infinitely variable speed transmission, which controls ratio control, while minimizing slippage and preventing component wear and tear.
The present invention combines the desirable feature of the continuously variable transmission with the non-slip characteristics of a gear drive. It achieves this combination with a cone drive geometry using a roller chain drive linkage continuously engaging sprockets. Variable ratios are adjusted continuously as the roller chain is moved laterally from the large diameter end of the cone to the small end or vice-versa. The latter action is similar to continuously variable transmissions using cone belt drives, but with the distinction that no slippage is possible in the present invention as the roller chain is positively engaged with sprockets, much the way gears are meshed, over the entire adjustment range.
Ratio changes can be made dynamically as in many continuously variable transmissions. The cone supports the roller chain on its surface, but it is two small keyed sprockets riding on spiral grooved shafts within the cone that protrude slightly through longitudinal slots through the cone surface that engage the roller chain.
As the ratio is changed by moving the plane of the roller chain laterally to engage a different cone diameter, the small keyed sprockets are moved with the chain, and the spiral grooves in their shafts fixed to the rotating cone rotate these keyed sprockets just the right amount so as to maintain proper phasing to perfectly engage the roller chain continuously.
A sprocket carrier element captures and aligns these small keyed sprockets in a vertical direction to engage the plane of the roller chain accurately. This carrier also is the element which moves the keyed sprockets laterally to change ratios, and it also resists further lateral movement once the ratio is set.
Since the roller chain is of a fixed size, the grooved input shaft with the drive sprocket is synchronously moved up or down relative to the cone shaft as required to maintain proper chain tension as the ratio is shifted.
While it is understood that either the cone shaft or the other shaft can be designated as input or output depending on the desired step-up or step-down ratio of an application, the single sprocket grooved shaft will be designated as the xe2x80x9cinputxe2x80x9d while the cone shaft will be the xe2x80x9coutputxe2x80x9d for sake of discussion of this invention.
Elements such as transmission housings and lateral or vertical adjustment mechanisms are not detailed in this invention as they are well known in the art. Adjustment mechanisms can take several forms including a hand- or motor-driven lead screw and nut, or a hydraulic cylinder. A more recently introduced device for position control is a servo controlled pneumatic cylinder with an auxiliary element using magneto-rheological fluid for added precision; this device can be used as well.
A secondary but equally important function of these lateral and vertical adjustment mechanisms is to resist forces arising out of normal operation that would tend to disturb the selected positions which correspond directly to the desired ratio.